The present invention relates to an adjustable optical phase-shifting device according to claim 1.
More particularly, the present invention relates to an adjustable optical phase-shifting device created for use in directional couplers, optical ring resonators, dispersion compensating devices, Mach-Zehnder interferometers, add-drop multiplexers, optical wavelength converters or amplitude-shift keying (ASK) as well as phase-shift keying (PSK) modulators operating particularly in low wavelength regions.
Optoelectronic integrated circuits made in thin-film technology often comprise phase-shifting devices used for the adjustment of the phase of an optical signal guided in a first waveguide relative to the phase of an optical signal guided in a second waveguide.
According to Govind P. Agrawal, Fiber Optic Communication Systems, Wiley Series in microwave and optical engineering, New York 1992, chapter 6.2.1, pages 232-234, optical signals can be modulated by means of a Mach-Zehnder interferometer comprising two arms wherein the phase of optical carrier signals is shifted relatively to each other according to electrical binary data. As long as the phase of the optical carrier signals, which originate from the same source, is identical, then the corresponding optical fields interfere constructively. An additional phase shift of adequate size introduced in one of the arms destroys the constructive nature of the interference of the optical carrier signals which are superpositioned on an output line of the ASK-modulator. The additional phase shift in the given example is introduced through voltage-induced index changes of the electro-optic materials (e.g. LiNbO3) used for said arms of the Mach-Zehnder interferometer as described in [2], Richard C. Dorf, THE ELECTRICAL ENGINEERING HANDBOOK, CRC Press LLC, Boca Raton 1997, chapter 31.3, pages 829-837.
Phase shifting devices are also used in directional couplers. An optical waveguide directional coupler filter with waveguides in LiNbO3 which is tunable with electrical control signals is disclosed in U.S. Pat. No. 4,146, 297.
In C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo and L. T. Gomez, Phase Engineering Applied to Integrated Optical Filters, IEEE Lasers and Electro-Optics Society, 12th annual meeting, San Francisco 1999, allpass filter rings and linear delay response architectures for dispersion compensations are described. A basic ring architecture consists of a tunable optical waveguide ring which is coupled to an optical waveguide through which optical signals are transferred. A thermo-optic effect is used to shift the phase of the signals within the ring. In order to obtain a desired filter response, it is critical to accurately fabricate the desired coupling ratio. To reduce the fabrication tolerances on the couplers and simultaneously to obtain a fully tunable allpass response, the basic ring architecture is preferably enhanced with a Mach-Zehnder interferometer (see FIG. 1). This enhanced ring structure, below called ring resonator, is briefly explained with reference to FIGS. 1 and 2.
FIG. 1 shows a prior art directional coupler with a first and a second waveguide 3, 4 aligned in parallel, with a first and a second coupler 301, 302, through which optical signals can be exchanged between said waveguides 3, 4, and with one thin film heater 100 covering a part of the first waveguide 3 lying between the couplers 301, 302. An optical signal entering the first waveguide 3 at port A will partially be coupled from the first coupler 302 to the second waveguide 4. Between the couplers 301, 302 the phase of the remainder of the optical signal transferred in the first waveguide 3 will be shifted according to the thermal energy applied to the first waveguide 3 by means of the thin film heater 100. The optical signal in the first waveguide 3 then interferes in the second coupler 302 with the optical signal of the second waveguide 4. Depending on the phase relationship between the optical signals the signal intensity in the second waveguide 4 will be increased or reduced accordingly.
In case that the second waveguide 4 is formed as a ring and enhanced with a thin film heater 101 for phase-shifting purposes, then the architecture shown in FIG. 1 corresponds to the tunable ring resonator shown in FIG. 2 respective, FIG. 1 which may be used for dispersion compensation.
In order to obtain a desired shift of the phase of the optical signal in the first waveguide 3 relative to the phase of the optical signal in the second waveguide 4 thermal energy provided by the thin film heater 100 is applied to the first waveguide 3 and not to the second waveguide 4. In the region of the thin film heater 100 the waveguides 3, 4 are therefore spaced apart at a distance which is sufficient to avoid a transfer of thermal energy from the thin film heater 100 to the second waveguide 4. Thermal energy provided by the thin film heater 100 is absorbed by the substrate 5 acting as a heat sink in such a way that the thin film heater 100 forces a temperature gradient with respect to the substrate 5.
Since the waveguides 3, 4 of the tunable ring resonator are kept apart from each other between the couplers 301, 302 over a relatively long distance, the architectures shown in FIGS. 1 and 2 are difficult to realise in small sizes as required for high frequency applications operating for example in the range of 25 GHz to 75 GHz.
As described above the temperature gradient will depend on the temperature and nature of the substrate 5 which may not be homogeneous over the whole circuit. With a change of the ambient temperature the operating conditions of the discussed circuit may vary considerably. Information regarding the temperature and hence the refractive index of the related waveguide is not provided by the circuit so that means for controlling the function of the circuit are limited. In addition shifting only the phase of the optical signal guided in the first waveguide of the device shown in FIG. 1 by establishing a temperature gradient between the heater 100 and the substrate 5 appears to be inefficient.
It would therefore be desirable to improve the described phase-shifting devices. It would be desirable to create an easily controllable phase-shifting device operating with high efficiency. It would be desirable to create a phase-shifting device which operates independently of changes of the ambient temperature. It would further be desirable to create a phase-shifting device which, besides the phase-shifting function, comprises a coupling function.
It would be desirable in particular to create a phase-shifting device for tunable ring resonators, directional couplers, add-drop multiplexers, Mach-Zehnder interferometers, optical filters, dispersion compensating devices, optical wavelength converters or amplitude-shift keying (ASK) as well as phase-shift keying (PSK) modulators suitable for operating in high frequency ranges.
It would also be desirable to control the adjusted temperature in order to reach and hold a selected phase shift in a narrow range.
It would further be desirable to create a phase-shifting device which in conjunction with related circuitry can be fabricated at reduced cost and in high packing density.
The above and other objects of the present invention are achieved by a device according to claim 1.
A phase-shifting device, which is electrically adjustable, is arranged on a substrate comprising at least a first waveguide and a thermoelectric element arranged adjacent to the first waveguide in order to shift the phase of an optical signal in the first waveguide by means of a thermo-optic effect according to a control voltage applied to the thermoelectric element which, according to the present invention, is a Peltier element comprising during operation a cold and hot side.
In a preferred embodiment of the invention the cold side of the thermoelectric element is arranged adjacent to the first waveguide and the hot side of the thermoelectric element is connected to a heat sink element.
In order to establish a temperature gradient between the first and a second waveguide in a further embodiment of the invention one thermal side of the thermoelectric element is arranged adjacent to the first waveguide and the other thermal side is arranged adjacent to the second waveguide. In this way the temperatures within the first and the second waveguide can efficiently and independently of external conditions be changed.
In integrated devices comprising an etched waveguide layer deposited on the substrate the electrically conducting segments are preferably placed upon and/or integrated into the cladding layer which is covering the waveguide layer.
In a preferred embodiment semiconductor parts of P- or N-type are diffused alternating into the cladding layer. In this case metal conductors preferably made of aluminum are used for connecting the corresponding ends of the semiconductor parts thus forming the thermoelectric element.
In another embodiment of the invention one thermal side of a first thermoelectric element is arranged adjacent to a first waveguide and the thermal side of opposite thermal polarity of a second thermoelectric element is arranged adjacent to a second waveguide with the other thermal sides of the thermoelectric elements being connected to each other with a heat conductive element. In this way the steepness of the temperature gradient established between the first and the second waveguide is increased thus increasing the sensitivity and range of the control function.
The response time of the phase-shifting device may be reduced by using two ore more thermoelectric elements in order to cool or warm a waveguide. This can easily be done by integrating differently doped semiconductor elements into the layers below and above the waveguide layer.
The solution allows to arrange the first and the second waveguide in the region close to the thermoelectric element in close proximity so that light energy of transferred optical signals is coupled from the first to the second waveguide.
Waveguides can therefore be aligned in the phase-shifting device in close proximity allowing to realise optical structures with higher densities. Since waveguides can be aligned in the phase-shifting device in close proximity the phase-shifting device can simultaneously operate as phase-shifting device and directional coupler.
The invention can be implemented advantageously in various optical circuits such as tunable ring resonators, directional couplers, add-drop multiplexers, Mach-Zehnder interferometers, optical filters, dispersion compensating devices, optical wavelength converters or amplitude-shift keying (ASK) as well as phase-shift keying (PSK) modulators suitable for operating in high frequency ranges.